The present invention relates to a method enabling a non-propagation threshold of fatigue cracks at high frequency to be determined for a blade of a turbine engine.
Conventionally, the sets of blades in turbojets (compressor blades and turbine blades) are stressed over a wide range of frequencies. Nevertheless, the critical frequencies are mainly those associated with the first modes in bending of the blades, which are generally situated above 500 hertz (Hz). In order to be able to dimension blades in fatigue, it is possible either to determine the dimensions of acceptable impacts on the blades, or else, more commonly, to have recourse in particular to a model for fatigue crack propagation that generally needs to be calibrated using experimental data. This data includes the threshold for non-propagation of a long crack, which threshold is written ΔKth and characterizes the threshold amplitude ΔK of the stress intensity factor (SIF) beyond which a crack propagates in fatigue.
Nevertheless, for a given material, this non-propagation threshold is determined almost exclusively in quasi-static manner, which is not representative of the frequency levels to which blades are subjected during their lifetime. Furthermore, certain materials such as Ti-6Al-4V or even TiAl can be sensitive to environmental effects, which can also give rise to this threshold being dependent on frequency.
At present, the non-propagation threshold is determined by methods in which a decreasing load is applied to a testpiece using hydraulic traction machines. The force imposed is then servo-controlled to the stress intensity factor, which means that it is necessary to know how the size of the crack is varying during the test. In order to be able to do this, various instrumentation devices exist that make it possible to determine the size of the crack during testing. A first such device relies on measuring a potential difference on the basis of a current flowing through the testpiece. As the crack propagates, the resistance that is obtained as a result of this potential difference measurement increases because of the reduction in the available section in the propagation plane. The second device relies on the “compliance” method in which a strain gauge is placed on the face of the testpiece opposite from the original location of the pre-crack, and when the crack propagates for given imposed force, an increase in the measured deformation can be observed. This effect results from a decrease in the stiffness of the testpiece in traction. With a master curve giving the relationship between stiffness and crack advance, it is possible to determine the size of the crack. A third device relies on the “comb” method in which numerous strain gauges are arranged so as to form a “comb” along the cracking path. When the crack reaches a strain gauge, the strain gauge breaks. Knowing the number of strain gauges that have been broken, it is possible to determine the size of the crack. Finally, still other devices rely on the correlation between images or optical measurements from which the position of the tip of the crack is estimated with the help of a telecentric system.
All of those conventional methods rely on using a device for determining the size of the crack during testing, thus making it possible in real time to adjust the stress intensity factor (SIF) that is applied to the edge of the crack (SIF=b*S*√(π*a) where b is the form factor of the crack, S is the applied stress, and a is the characteristic dimension of the crack). A decrease in the SIF is thus applied and the threshold is considered as being reached as soon as the speed of advance of the crack (as measured during the test) becomes less than a given value, e.g. 10−10 meters per cycle (m/cycle), as proposed in the ASTM E647 standard.
Apart from the fact that the threshold is always characterized under quasi-static conditions (1 Hz to 30 Hz), and therefore on the assumption that the non-propagation value that is obtained is constant as a function of frequency, which is not true, the major drawback of those conventional methods is that they require the size of the crack to be determined in situ. Devices for measuring potential can be applied only to a material that is conductive and they require a data acquisition and processing system that is fast in order to be able to perform regulation. As a general rule, this is not possible at high frequencies such as those to which parts are subjected in operation and for which it is desired to characterize the materials of which they are made (>500 Hz). The compliance method can be applied to any homogeneous material, but it likewise requires a fast data acquisition and processing system in order to perform regulation, which is likewise incompatible with the frequencies at which it is desired to perform measurements. The comb method does not necessarily lead to effective servo-control. The length of the crack can be determined in discrete manner only, i.e. at the locations where the strain gauges are positioned. It can then happen that overloading effects occur at the tip of the crack. Finally, optical measurements can be used only when the propagation speed of the crack is sufficiently slow. Between two measurements, the variation in the length of the crack must be small in order to avoid the effect of overloading the tip of the crack.